In order to see this glitch, you must play in two-player co-op, and player two must be dead. Play the level until you get to the part where you chase the Heretic Leader in the Banshee. Kill player two, then, if needed, kill all enemies. After that, locate the arrow that points to the place where you must go to continue the level. You cannot come back and finish the level after you fly up; you have to restart at the last checkpoint. Additionally, it is possible to crouch while in the air.